Summer is arriving. We’ve had lots of sunshine and thunderstorms with rain these days. For sunshine, you protect yourself with lots of sun screen with a high protection factor. If it rains, you put on a raincoat and wellies. So, if you want to protect what is dear to you, you cover it with the appropriate cover. Hmm, if your pump gets eroded by your mixture, you cover it with a protective layer. Right?
So, let’s see what options we have? Common solutions to protect the wear parts of the pump are:
Vulcanise a rubber film over a new cast wear part. Usually, the pump parts are designed to receive an additional layer of several mm to a couple of cm. Astoundingly, the soft rubber, lasts longer than the hard alloy. This is due to the elasticity of the rubber. Impacting particles are bouncing back into the fluid and don’t damage the metal1. There have been several developments, where polyurethane2 can be a viable alternative to rubber with the same protective principle. Rubber and polyurethane have to be applied by specialised companies under controlled conditions. One warning though, rubber can be cut. When dredging shells or coral, the rubber is sliced to pieces and the flow peels away sheets of rubber or PU.
Instead of the flexible rubber, also hard chemical compounds have been developed to be applied as protective layer. This layer can be applied on a virgin part, that can accommodate the layer by design, just like the rubber cover. Or it can be used to restore an already worn down part and extend the lifetime that way. The pastes have been engineered to be applied in the field: wet, rust, salt and dirt. As it easily applied, many owners are very fond of this solution. Although lifetime is not extended as much as the first option.
Another process for new builds and restoration is hard-facing3. On a suitable base alloy, the vulnerable surface is cladded by welding a layer of very hard metal onto it. The hard-facing can be much harder than the sand particles (Mohs remember) and that the added material is brittle as glass doesn’t matter as it is carried by a much more ductile base material. As the base material is also softer, once the hard layer is away, the wear rate will become unexpectedly rapid. Especially as such local wear spots tend to eat through the material, due to the increased turbulence. Special care should be taken, when the wear part is not completely covered in hard-facing, but only partially covered. The discontinuities are hot spots of wear. Discontinuities can also appear from the hard-facing itself. The different material properties of cladding and base material cause to be vulnerable to check cracking and flaking4.
All three solutions are labour intensive. especially vulcanising and hard-facing takes many hours. And applying the wear paste has to be done more often. Even if labour is cheap in your operation, you still have to take it into account in your spare part strategy. More so, if you only rely on just one wear part and don’t have one on stock and the dredge is idle, you’ll lose a lot of money on income.
Another consideration might be the environment. An all metal wear part can be recycled easily. Rubber, PU and paste can be a pain to get rid of, responsibly.
Off course, I am an engineer at the manufacturing side and the above perspective may be biased, but I like to be proven wrong. Until then, I would rather purchase a durable all metal wear part, than go to the trouble and costs of the extra handling. Whatever your final wear part strategy, it should revolve around having the correct spare parts at the right time at the right location.
A dredge company makes its profit by economically transporting sand by mixing it with water. Unfortunately, this happens to be the best combination to literally ‘sand blast’ steel. Every effort should be made to reduce the wear and tear on the dredging components and especially the dredge pump. If not tenderly cared for, your dredge pump may erode away. Performance and profit will follow down the drain, also.
The background of the wear on the dredge components, is scratching. The small hard particles in the sediment are blasted against the surface of the wear parts. As the sand is usually harder than the steel, the steel gets scratched. Enough scratches on top of each other makes the wear. The principle of scratching different materials against each other was scientifically explored by Friedrich Mohs. Although the effects were already known by the ancient Greeks1. Mohs proposed a hardness scale, that is very practical and will give you a first estimate of the hardness2.
On the left side are the classical Mohs minerals, that we also sometimes encounter in dredging. On the right side there is also a suggestion of tool material that is of comparable hardness. If you need to scratch on the mineral on the left, you need at least a tool of the corresponding hardness on the right. Quartz is a main component of sand. And from the scale, you can see, that a normal steel nail will not be tough enough to make a scratch. And that is exactly what we see in dredging. Wear parts for handling the soil are usually made of sophisticated alloys to be harder than sand. The wear rate reduces significantly beyond the hardness of sand. In selecting wear parts material3, we usually discuss the wear index. This is the factor in which a certain material lasts longer than normal construction steel under the same conditions.
There is a trend: harder material lasts longer. Off course it is very attractive to select the hardest material. But, there are two considerations:
Hard material tends to be very brittle. For an uniformly distributed sediment with no heavy lumps (read: rocks) this might be fine. As soon as rocks and debris are involved, the wear part might crack due to impact from stones etc.
Hard material tends to be expensive. It requires exotic elements to cast and extensive treatment and machining to finally reach the required hardness.
Still, the harder material might be your choice. If the best material at 10 times the wear index, is three times as expensive as the softer cheap material, your will end up with a three times better wear rate per Euro (or Dollar). And it is not an investment. Wear parts are consumables.
Speaking of money, wear parts do involve some financial planning. At best, the contractor has his own stock. In case of a worn wear part, the part is immediately available. Though this requires some investment indeed. We do have a stock for emergency deliveries, but transport costs time also. Let alone, if the part has to be cast. Casting is a laborious process that can take 16 to 20 weeks. Even with all the modern progress, we are still limited by the physical processes involved. But the casting process itself sure looks quite spectacular!
These days I have been very busy drafting a manuscript about our ¡VAMOS! project results1 for a dredging conference paper. As every writing process, there is so much to tell and so little space available. At a certain moment there follows a phase called: ‘kill your darlings’. You have to scrap parts that contribute less to the main message of the article. Still some of those orphans are worth sharing. So here is a part from the paper that might be interesting for you.
For those unfamiliar with ¡VAMOS!, it is a Viable Alternative Mining Operating System2, where we are cooperating in a 17 partner consortium to develop equipment and procedures for exploiting mineral reserves in disused or currently unavailable mines in the EU3. Many mines are disused, but still contain some reserves, that are unrecoverable due to an uneconomic stripping ratio4.
We developed a prototype mining vehicle (MV) and an accompanying launch and recovery vessel (LARV)5. Although the requirement for the slurry circuit on the MV are deceivingly similar to a normal dredge system, there is one fundamental difference in character: vertical transport. At the system architecture phase we assumed a dredging depth of at least 100 m. For clean water, this poses not so much of a problem, once pumping mixture is where the geodetical height difference comes into play. At 100 m a 1500 kg/m³ mixture requires an additional 5 bar of static head.
The dredge pump has to cope with the large variation in head requirements. For the prototype machine, the only option is to vary the dredge pump speed. Still at a normal operating condition, we expected a head requirement for 10 to 16 bar. This is why we developed a dual stage dredge pump, it delivers double the head of a normal dredge pump.
The variation of the pump speed has been accomplished by various controllers working together. The power is generated on the LARV by a diesel engine driving a generator. A frequency drive supplies a hydraulic power pack on the MV. The power pack has a variable displacement pump for controlling the flow. At the dredge pump side there is also a variable displacement motor.
With this setup in place, the dredge pump can vary between a slurry circuit just filled up with clean water and a fully filled riser with heavy mixture at the operating point. On top of that, there will always be the possibility, that the density increases even more. The flow will reduce and so will the power consumption. This enables the drive to increase the speed for extra oompf of the dredge pump to clear the riser. Where normally the dredge pump speed is controlled by the pump swash plate, the motor swash plate is so to speak the turbo boost. This is similar to a constant power drive for normal dredges, but the vertical riser makes the problem more pronounced.
So, not only the starting up of a dredge pump should be considered in the design of the drive train, but also the variations in operating point. Regarding the comments I received on my last post, yes indeed a production model of the ¡VAMOS! system would have an all-electric drive. Just be sure to have enough copper in the motor to cover every possible operating point.